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ITER monoblock performance under lifetime loading conditions in Magnum-PSI
No full textThe ITER divertor will be exposed to extremely high plasma fluences over its lifetime, and it is known that plasma exposure can lead to a variety of particle-induced surface-morphology and microstructure changes in tungsten. However, no data exists at fluences comparable to those expected over extended ITER operations (10 30−31 m−2) and so it is uncertain how these changes will evolve and affect the divertor performance over such long timescales. Six monoblocks were exposed to high flux plasma comparable to partially-detached plasma conditions in the ITER divertor in Magnum-PSI. Different exposures used different plasma species (H, He, D or D + He) and aimed to replicate conditions similar to those during different phases of the ITER staged approach. The highest fluence achieved was 10 30 D m−2, comparable to around one year of ITER Fusion Power Operation. Post-mortem analysis by Nuclear Reaction Analysis revealed very low deuterium retention throughout the blocks, while surface analysis showed no cracking or damage, but did observe helium fuzz growth at low ion energies of 8–18 eV, below typically assumed ion energy requirements for such growth to occur. Metallographic sectioning revealed recrystallization up to 2.2 mm below the surface of monoblocks exposed at peak surface temperatures of up to 1580 °C for different durations up to ~20 h. Finite Element Method analysis coupled to metallographic and Vickers Hardness identification of the boundary of the recrystallized region identified a faster recrystallization process compared to literature expectations, reinforcing that recrystallization dynamics is an important criterion for tungsten grade selection for the ITER divertor. Overall, no major damage or failure was identified, indicating that the design is capable of fulfilling its steady-state performance requirements under high flux, high fluence plasma loading conditions in the ITER divertor
Growth mechanism of subsurface hydrogen cavities in tungsten exposed to low-energy high-flux hydrogen plasma
No full textDue to a lack of direct experimental results, the detailed mechanisms that govern the blistering behavior of tungsten (W) exposed to ITER-relevant condition in nuclear fusion remain unclear. The growth mechanism of hydrogen (H) blisters is one example. In this work, recrystallized W was exposed to H plasma at 50 eV, 1.5×1026m−2, and 573 K. Transmission electron microscopy (TEM) samples were prepared using plasma-focused ion beam (FIB) followed by flash-polishing to effectively remove surface damages induced by FIB. The TEM images revealed that the general blisters observed on the exposed surface are associated with underlying cavities. A considerable amount of dislocations were found in the vicinity of the cavities. Prismatic dislocation loop arrays were observed, including small size \u27coffee-bean\u27 prismatic loops and large size prismatic loops. Near the tip of surfaces cavities, evidences for the emission of shear loops were also found. Based on the experimental findings, a multi-stage growth mechanism of H cavities was proposed. The loop-punching mechanism is operative for both very small cavities and cavities with sizes larger than several hundreds of nanometers. Whereas at intermediate sizes, cavities grow by emitting shear loops from the cavity tip
Low-temperature, atmospheric pressure reverse water-gas shift reaction in dielectric barrier plasma discharge, with outlook to use in relevant industrial processes
No full textPlasma discharges offer a direct way to convert electrical to chemical energy and to store volatile renewable energy sources. Converting CO2 in this way can contribute to reducing the greenhouse effect, and provide additional opportunity for chemical processing, e.g., on-site or on a small scale. The CO2 hydrogenation to CO via the reverse water-gas shift reaction (RWGS) generates synthesis gas for use as feedstock to different fuels and chemicals. The RWGS reaction carried out in a Dielectric Barrier Discharge (DBD) plasma reactor benefits from operation at ambient pressure and mild temperature, as compared to the harsher conditions of conventional RWGS processing. To develop that with outlook to real-life uses, e.g., toward methanol synthesis, key performances need to be achieved; that is, i.a., a threshold CO2 conversion, a high CO selectivity at low impurity (low CH4 selectivity), and a high (H2 − CO2)/(CO + CO2) ratio (favourable for high reaction rates) as well as tolerable energy efficiency. Central plasma process parameters for this are the feed gas ratio, residence time, and uniformly distributed microdischarges. The optimisation of an individual key performance can be adverse to the other so that the process exploration is a task. This gives room to introduce new plasma operation types, and the burst mode was applied for the first time to the RWGS reaction in the present work. By this fast (millisecond) periodic switching on and off the plasma, the process temperature can be reduced as well as a better microdischarge distribution can be achieved. The residence time is not only set by the flow rate, as commonly done, but also by taking the discharge gap as an additional parameter of freedom, which also impacts the reducing distribution. As a result of relevant process conditions, at CO selectivity of 98%, 337 mmol/kWh is obtained as the energy efficiency of CO formation. Whereas the best CO2 conversion of 50% and the (H2 − CO2)/(CO + CO2) ratio of 2 were obtained at respective optimum process parameters
The effect of space charge on blocking grain boundary resistance in an yttrium-doped barium zirconate electrolyte for solid oxide fuel cells
No full textCO2 Conversion in Nonuniform Discharges: Disentangling Dissociation and Recombination Mechanisms
Get PDFMotivated by environmental applications such as synthetic fuel synthesis, plasma-driven conversion shows promise for efficient and scalable gas conversion of CO2 to CO. Both discharge contraction and turbulent transport have a significant impact on the plasma processing conditions, but are, nevertheless, poorly understood. This work combines experiments and modeling to investigate how these aspects influence the CO production and destruction mechanisms in the vortex-stabilized CO2 microwave plasma reactor. For this, a two-dimensional axisymmetric tubular chemical kinetics model of the reactor is developed, with careful consideration of the nonuniform nature of the plasma and the vortex-induced radial turbulent transport. Energy efficiency and conversion of the dissociation process show a good agreement with the numerical results over a broad pressure range from 80 to 600 mbar. The occurrence of an energy efficiency peak between 100 and 200 mbar is associated with a discharge mode transition. The net CO production rate is inhibited at low pressure by the plasma temperature, whereas recombination of CO to CO2 dominates at high pressure. Turbulence-induced cooling and dilution of plasma products limit the extent of the latter. The maxima in energy efficiency observed experimentally around 40% are related to limits imposed by production and recombination processes. Based on these insights, feasible approaches for optimization of the plasma dissociation process are discussed.</p
Plasmon-driven synthesis of individual metal@semiconductor core@shell nanoparticles
No full textMost syntheses of advanced materials require accurate control of the operating temperature. Plasmon resonances in metal nanoparticles generate nanoscale temperature gradients at their surface that can be exploited to control the growth of functional nanomaterials, including bimetallic and core@shell particles. However, in typical ensemble plasmonic experiments these local gradients vanish due to collective heating effects. Here, we demonstrate how localized plasmonic photothermal effects can generate spatially confined nanoreactors by activating, controlling, and spectroscopically following the growth of individual metal@semiconductor core@shell nanoparticles. By tailoring the illumination geometry and the surrounding chemical environment, we demonstrate the conformal growth of semiconducting shells of CeO2, ZnO, and ZnS, around plasmonic nanoparticles of different morphologies. The shell growth rate scales with the nanoparticle temperature and the process is followed in situ via the inelastic light scattering of the growing nanoparticle. Plasmonic control of chemical reactions can lead to the synthesis of functional nanomaterials otherwise inaccessible with classical colloidal methods, with potential applications in nanolithography, catalysis, energy conversion, and photonic devices
Plasma induced vibrational excitation of CH4-a window to its mode selective processing
No full textVibrational excitation of methane is believed to promote chemistry and improve product selectivity, compared to thermal conversion methods. We report on unique direct measurements of vibrational–rotational non-equilibrium in methane plasma. The non-equilibrium is sustained for 50 μs, after which the gas temperature equilibrates with the vibrational temperature at around 900 K. The plasma is generated by applying 200 μs, 30 Hz pulses of microwave radiation to methane at 25 mBar. We demonstrate that in microwave discharges, power transfer to vibrational modes of CH4 is the dominant power transfer mechanism, which leads to creation of a vibrational–translational (VT) non-equilibrium. VT relaxation is determined to be the dominant translational heating mechanism in the discharge. However, the high electron temperature at breakdown also leads to strong electronic excitation which may be responsible for some of the heating. Furthermore, we find that the CH4 vibrational levels are in equilibrium with each other due to fast intra-polyad relaxation (VV), and therefore bending vibrational modes population density is greatly in excess of stretching vibrational modes. The window of opportunity to exploit this non-equilibrium is limited by the VT relaxation timescale, which is approximately 50 μs in our experiment
Environmental impact assessment of plasma-assisted and conventional ammonia synthesis routes
No full textThe importance of ammonia in the fertilizer industry has been widely acknowledged over the past decades. In view of the upcoming increase of world population and, in turn, food demand, its production rate is likely to increase exponentially. However, considering the high dependence on natural resources and the intensive emission profile of the contemporary ammonia synthesis route, as well as the rigid environmental laws being enforced at a global level, the need to develop a sustainable alternative production route becomes quite imperative. A novel approach toward the synthesis of ammonia has been realized by means of non‐thermal plasma technology under ambient operating conditions. Because the given technology is still under development, carrying out a life cycle assessment (LCA) is highly recommended as a means of identifying areas of the chemical process that could be potentially improved for an enhanced environmental performance. For that purpose, in the given research study, a process design for a small‐scale plasma‐assisted ammonia plant is being proposed and evaluated environmentally for specific design scenarios against the conventional ammonia synthesis employing steam reforming and water electrolysis for hydrogen generation. On the basis of the LCA results, the most contributory factor to the majority of the examined life cycle impact categories for the plasma‐assisted process, considering an energy efficiency of 1.9 g NH3/kWh, is the impact of the power consumption of the plasma reactor with its share ranging from 15% to 73%. On a comparative basis, the plasma process powered by hydropower has demonstrated a better overall environmental profile over the two benchmark cases for the scenarios of a 5% and 15% NH3 yield and an energy recovery of 5% applicable to all examined plasma power consumption values
Tuning the Optical Characteristics of Diketopyrrolopyrrole Molecules in the Solid State by Alkyl Side Chains
No full textUnderstanding the Impact of Different Types of Surface States on Photoelectrochemical Water Oxidation: A Microkinetic Modeling Approach
Get PDFThe oxygen evolution reaction (OER) has been identified as one of the performance-limiting processes in solar water splitting using photoelectrochemical (PEC) cells. One of the reasons for the low OER performance is related to the existence of different types of surface states at the semiconductor–electrolyte interface: recombining surface states (r-SS) and surface states due to intermediate species (i-SS). Since the impact of surface states on OER is still under debate, we investigate how different types of surface states affect PEC water oxidation and how they impact experimental measurements. In a new computational approach, we combine a microkinetic model of the OER on the semiconductor surface with the charge carrier dynamics within the semiconductor. The impact of r-SS and i-SS on the current–voltage curves, hole flux, surface state capacitance, Mott–Schottky plots, and chopped light measurements is systematically investigated. It is found that (a) r-SS results in a capacitance peak below the OER onset potential, while i-SS results in a capacitance peak around the onset potential; (b) r-SS leads to an increase in the OER onset potential and a decrease in the saturation current density; (c) r-SS leads to Fermi-level pinning before the onset potential, while i-SS does not result in Fermi-level pinning; and (d) a smaller capacitance peak of i-SS can be an indication of the lower catalytic performance of the semiconductor surface. Our approach in combination with experimental comparison allows distinguishing the impact of r-SS and i-SS in PEC experiments. We conclude that r-SS reduces the OER performance and i-SS mediates the OER.</p